Method of electrostatic electrophotography



1954 L. B. BUTTERFIELD 2,693,416

METHOD OF ELECTROSTATIC ELECTROPHOTOGRAPHY Filed May 19, 1950 I ATTORNEYUnited States Patent Ofi" 2,693,416 Patented Nov. 2, 1954 iVmTl-IOD OFELECTROSTATIC ELECTRO- PHOTOGRAPHY Louis Boydston Butterfield,Plainfield, N. J., assignor to Western Electric Company, Incorporated,New York, N. Y., a corporation of New York Application May 19, 1950,Serial No. 163,007 6 Claims. (Cl. 95-1.9)

This invention relates to electrostatic electrophotography and moreparticularly to a method for reproducing images from an image bearingsheet of material.

Previous methods for reproducing images by use of electrostaticelectrophotographic processes may be summarized broadly as processeswherein an invisible electrostatic image is formed on a surface ofphotoconductive material and then a visible powder is sprinkled on thephotoconductive surface to develop a visible image from the latentelectrostatic image. The next step is to transfer the visible powderimage from the photoconductive surface to a sheet of paper and then tofix permanently the image to the paper by heat.

It is an object of this invention to provide a simplified method ofelectrostatic printing wherein the steps necessary to carry out theprocess are reduced.

It is an additional object of this invention to improve methods foreconomically reproducing designs, configurations or outlines of solidobjects.

A further object of this invention is to provide a method for producingan electrostatically charged image on a sheet of insulating material inaccordance with a light image projected onto the sheet of material.

Another object of the invention contemplates a method wherein anelectrostatically charged image acts through a transparent ortranslucent sheet of material to attract powder to form a visible image.

A still further object of this invention is to provide a method forprojecting a light image through a transparent or translucent sheet ofmaterial to form an electrostatic image on a photoconductive surface.

In accordance with one embodiment of the present method, a film havingan image to be reproduced is placed in front of a light source. Theimage on the film is projected by a beam of light through anelectrostatically charged transparent or translucent sheet of materialonto a photoconductive layer whereupon the portions of thephotoconductive layer subjected to the light are rendered conductive.Charges of opposite polarity to those on the transparent or translucentmaterial now migrate, by attraction, through the lighted portions of thephotoconductive layer to the under side of the transparent ortranslucent sheet of material to form electric couples which havenegligible external fields. Thus the charges on the portions of thetransparent or translucent sheet of material not subjected to lightremain with an effective external field. The transparent or translucentsheet of material is then sprinkled with an electrostaticallyattractable visible powder to form a visible image of the electrostaticimage. The sheet of material is then removed from contact with thephotoconductive surface and finally the powder is fixed by any suitableprocess to the sheet of material to form a permanent visible image.

Other features and advantages of the invention will become apparent fromthe following detailed description taken in conjunction with theaccompanying drawings in which:

Fig. 1 is a perspective view illustrating apparatus suitable for use incharging a photoconductive plate in accordance with the invention;

Figs. 2, 3 and 4 are views showing apparatus employing various methodsof projecting an image through a sheet of material onto thephotoconductive element;

Fig. 5 is a sectional view illustrating an arrangement of apparatus forapplying powder onto an electrostatic image bearing sheet of material;

' Fig. 6 is a perspective view showing the removal of aelectrostatically charged image sheet of material bearing a reproducedpowdered image from the photoconductive plate;

Fig. 7 is a perspective view illustrating apparatus for permanentlyfixing a powdered image to the sheet of material;

Fig. 8 is a perspective view disclosing an alternative arrangement ofelements suitable for use in electrostatically charging thephotoconductive plate.

Referring to the drawing wherein like reference numerals designate thesame elements throughout the several views, there is shown in Fig. 1 ametallic plate 10 having coated thereon a thin photoconductive layer 11.The metallic plate 10 may be made of copper, aluminum or any othersuitable conductive or semi-conductive material which does notchemically react perniciously with the photoconductive layer 11.

Any one of a variety of photoconductive materials may be used for thelayer 11 such as amorphous selenium, sulphur, germanium, lead sulphide,copper oxide, silver chloride, silver iodide and various combinations ofselenium and sulphur. The term photoconductive material as used in thisapplication includes those materials having such a change in theirelectrical characteristics in the transition from darkness to light thatan electrostatic image can be produced and maintained by said materialsduring an electrostatic printing cycle.

A sheet of transparent or translucent electrically insulating material12 is placed on top of the photoconductive layer 11. This sheet ofmaterial is made of any suitable electrically insulating material suchas paper, cloth, or cellulose acetate, polystyrene or other plastics. Itis upon this sheet of material that an image is to be reproduced.

A battery 13 or other suitable source of D. C. energy is connected by awire 14 to the under side of the metallic plate 10. A corona dischargewire 15 is positioned above the sheet of material 12 and in closeproximity thereto. This corona discharge wire is connected by aconductor 16 to the battery 13. It is preferable that the source of D.energy be suitable to maintain a potential of 5,000 to 10,000 voltsbetween the corona discharge wire 15 and the metallic plate 10.Therefore, as the metallic plate 10, together with the selenium layer 11and the sheet material 12 are reciprocated beneath the wire 15 adischarge takes place in the region between the corona discharge wire 15and the sheet of insulating material 12. This particular procedure iscarried out in a darkened space and as a result, the charges produced bythe corona wire 15 are retained on the surface of the overlying sheet ofinsulating material 12.

Referring now to Fig. 2, a fihn to be reproduced is placed on top of thesheet of transparent or translucent material 12. The assembled film 17,transparent or translucent sheet of material 12, photoconductive layer11 and metallic plate 10 are positioned with respect to a light source18 arranged to uniformly illuminate the film 17. The light readilypasses through those portions of the film 17 not having the image andthrough the sheet of transparent or translucent material 12 onto thephotoconductive layer 11. Upon the light striking the photoconductivelayer 11, the layer 11 is illuminated in the outline of the image to bereproduced. The portions of the photoconductive layer 11 subjected tothe light are rendered more conductive thereby allowmg electrostaticcharges of opposite polarity with those on the surface of the sheet ofinsulating material 12 to migrate through the photoconductive layer 11to the under side of the sheet of insulating material 12 to formelectric couples with the charges on the outer surface. These electriccouples will be formed over the entire area subjected to light and havea negligible external field. An remains on those portions of the surfaceof the sheet of insulating material 12 which are not subjected tosuflicient light to render conductive the underlying layer ofphotoconductive material 11. There is thus obtained on the transparentor translucent sheet of insulating material 12 an electrostatic latentimage corresponding to the image contained on the film 17.

It is to be understood that original image bearing materials other thanfilms may be used to produce the light image. Among the various types oforiginals that may be satisfactorily used to produce a light image aretype- 17 hearing an image 3 written pages of transparent or translucentpaper, drawings on tracing paper or cloth, designs contained on transparent or translucent sheets of plastic material, solid objects, theoutline of which is desired to be reproduced, etc.

Fig. 3 illustrates another method of exposing thephotoconductive layer11 to a light source 20. The image bearing film 21 or other materialcontaining a configuration to be reproduced is placed beneath the lightsource 2'0 and the image is projected through a lens 22 onto thetransparent or translucent sheet of material 12. The light passesthrough the transparent or translucent. sheet of material 12 to renderthe photoconductive layer 11 conductive in. the outline of the image tobe reproduced. The portions. of the photoconductive layer 11,. renderedconductive by the light, permit the migration of charges of opposite.polarity to the under side of the sheet of insulating material 12 toneutralize the eifect of the charges on the outer surface of the sheetof insulating material in the outline of the light image. Therefore, anelectrostatically charged image with an efiective external field remainson the surface of the sheet of insulating material 12. This chargedimage is a reproduction of the image on the film 21..

Fig. 4 illustrates an alternative method of projecting the image ontothe photoconductive layer 11. A source of light 3%) projects light ontoan image bearing surface 31 to reflect the image through a lens 32 andthrough a sheet of transparent or translucent electrostatically chargedinsulating material 12 onto the photoconductive layer 11. The imagebearing surface 31 in this instance need not be made of transparent ortranslucent material since opaque material serves. to reflect adequatelythe light to. produce the desired light image. The areas on thephotoconductive layer 11 not sufficiently illuminated by light,corresponding to the image on the sheet, 31, remain non-conductive. Theremaining portions of the photoconductive layer 11 subjected to. lightare rendered conductive to permit. the migration of charges to the underside of the sheet of insulating material 12. Inasmuch as these migratingcharges are of opposite polarity with the charges on the outer surfaceof the sheet of insulating material 12, electric couples in the outlineof the light image are formed which render non-effective the externalfield of the charges subjected to light leaving an effectiveelectrostatically charged image in the outline of the image on thesurface 31..

Upon exposure of the photoconductive layer 11 in any of the mannersshown in Figs. 2, 3 and 4, an electrostatic latent image, is. producedon the transparent or translucent sheet of insulating material 12. Thislatent image may be made. visible by sprinkling finely divided powder 35over the surface of the transparent or translucent material 12 asillustrated in Fig. 5. This powder 35' may be of any type depending uponthe type of. finished print desired. Pulverized resins. of many vari--eties may be used; however, resins which can be. melted or madeadhesive. by heating are preferred. The powder 35 passing over thetransparent or translucent material 12 is attracted to and adheres. tothe sheet of material. in the outline of the electrostaticallychargedimage contained on the surface 12.

The sheet of transparent or translucent material 12. 7

now hearing the powdered image, as shown in Fig, 6,, is removed from thephotoconductive layer 11 and heated by any of a number of arrangements,such as the radiant heater 49, illustrated in Fig. 7. The heat acts tofix permanently or set the powdered image 37. to the sheet oftransparent or translucent material 12 by melting or. rendering thepowder adhesive so. that: it is retained on.

the sheet 12. Other means may be. used to secure the.

powder on the sheet, 12. such as by spraying; the pow-- dered image witha. glue, lacquer or fluid adhesive com-- pound.

In Fig. 8. there is illustrated an alternative method of charging thephotoconductive. layer 11.. In this. instance, the sheet of,transparent. or translucent material is. not placed in contact. withthe. photoconductive layer 11' during the charging operation bythecorona discharge wire 15., The photoconductive layer 11 is charged byreci'pro-- eating the layer 11 and the metallic plate 10 beneath thecorona dischargewire 15. This procedure is carried out in a darkenedspace inorder that the photoconductive layer remains non-conductivetoretainthecharge on-sthe surfaceof the photoconduct-ive layer. e

The next step in this alternative method is to place a sheet oftransparent or translucent insulating material onthe surface of thephotoconductive layer 11. The film 17 is next placed on the sheet oftransparent or translucent insulating material and then the assembledfilm 117, sheet of insulating material 12, together with the seleniumcoated metal plate 1% are subjected to a source of light as illustratedin Fig. 2. Upon the light striking the photoconductive layer 11, thelighted surface portions of the layer 11 are discharged through the nowconductive portions of the photoconductive layer 11, through theconductive plate 16 and thence to ground 19. There remains on the outersurface of the photoconductive layer 11 an electrostatically chargedlatent image having a configuration identical with the image on the film17.

The electrostatic latent image may also be produced on thephotoconductive layer 11 by the methods dis- .closed relative to Figs. 3and 4. The effective electrostatic field of the charged latent imageacts through the sheet of transparent or translucent insulatingmaterialto attract the electrostatically attractable powder toform apowdered image of the latent charged image. The visible image is thenfixed on this sheet of insulating material by the method disclosed withregard to Fig. 7.

it is to be understood that the above-described methods and arrangementsare simply illustrative of the application of the principles of theinvention, and that many other modifications may be made withoutdeparting fromthe invention.

What is claimed is:

l. A method of electrostatic electrophotographic reproduction of imageswhich comprises electrostatically charging the outer surface of a sheetof insulating material while its under surface is in contact with aphotoconducti-ve layer, producing an electrostatic image on said outersurface of said sheet by exposure to a light image, applyingelectrostatically attractable powder to the charged outer surface ofsaid sheet, removing said sheet from said photocond'uctive layer, andthen permanently affixing the powder to said sheet of insulatingmaterial.

2. A method of reproducing indicia on a sheet of light transmittinginsulating material, which comprises producing an eifectiveelectrostatic field over the entire outer surface of a sheet of lighttransmitting insulating material, while the inner surface. of said sheetis in contact with a layer of photoconductive material, projecting alight image of the indicia to: be reproduced through said sheet ontosaid photoconductive layer to neutralize the portions of theelectrostatic field subjected to. light, applying" electrostaticallyattractahle powder to said outer surface of' said sheet to develop avisible powdered image in the area of the remaining effectiveelectrostatic field, separating said sheet. from said layer, andthereafter fix i'rfig' the powdered image tosaid outer surface of said seet.

3. The method of making a reproduction of an image which comprises:placing a sheet of light transmitting insulating material in contactwith a photoconductive layer on an electrically conductive plate,advancing the material, photoconductive layer and electricallyconductive plate through an electrostatic field to place anelectrostaticcharge on the outer surface of the light transmitting material,subjecting the image to be reproduced to a source of light to form alight image, projecting the light image through the light transmittingmaterial onto the surface of the photoconductive layer to render thelighted portions of the photoconductive layer conductive to produce acharged image on the outer surface of the light transmitting materialcorresponding to the projected light image, dusting the outer surface ofthe light transmitting material with afinely divided electrostaticallyattracta-ble powder to form apowdered image of the. electrostatic image,removing saidv sheet. from said photo'- conductive layer, and thenpermanently fixing the powder to the sheet of light transmittingmaterial.

4 The method of reproducing a design from a sheet of material: having adesign thereon which comprises positioning a sheet of light transmittinginsulatingmaterial in contact with a layer of photoconductive material,elect-rostatically' charging the surface of the light transmitting'material, reflecting light off the surface of the design bearing sheetof material to form a light image of the design, projecting the lightimage through the light transmitting insulating material onto thephoteconductive layertorender the portions of the photoconductive layersubjected to light conductive to permit charges to migrate to the underside of the sheet of light transmitting insulating material toneutralize the charge on the outer surface of the light transmittinginsulating material in the outline of the design, developing theportions of the photoconductive layer which remain charged by placingelectrostatically attractable powder on the said charged portions of thesheet of light transmitting insulating material, separating the sheet oflight transmitting insulating material from the photoconductive layer,and permanently fixing the powder to the sheet of light transmittinginsulating material.

5. The method of electrostatic electrophotography wherein a sheet ofmaterial whose conductivity varies with illumination is placed with oneside in contact with a sheet of inert conductive material, a sheet oftrans lucent insulating material is placed in contact with the otherside of the variable-conductance material, the outer surface of saidtranslucent insulating material is electrostatically charged in the darkby exposing said insulating material to an electrical field produced bya potential of from approximately 5,000 volts to approximately 10,000volts, an image whose configuration is desired to be reproduced isplaced between a potential source of illumination and the threesuperimposed sheets, the source of illumination is energized to projectlight rays around the image so that the configuration thereof willappear on the charged outer surface of the translucent insulatingmaterial, said light rays passing through the translucent material tothe variable-conductance material, thereby increasing the conductivityof the variable-conductance material and causing the charge on the outersurface of the insulating material corresponding to the illuminatedareas of the variable-conductance material to be rendered ineffectivewhereby an electrostatic latent image is formed on the outer surface ofthe translucent insulating material, applying electrostaticallyattractable powder to the outer surface of the sheet of translucentinsulating material to form a visible powder image thereon, separatingthe sheets of translucent insulating material and thevariable-conductance material from each other, and then permanentlyfixing the powdered image to the sheet of translucent insulatingmaterial.

6. The method of electrostatic electrophotographic reproduction ofimages which comprises the steps of coating a metallic member with aphotoconductive material, placing light transmitting insulating materialon said coating of photoconductive material, subjecting the coatedmetallic member with said light transmitting insulating material thereonto an electrostatic field to place an electrostatic charge on the outersurface of said light transmitting insulating material, causing a lightimage of the image to be reproduced to be projected on the outer surfaceof ductive material, dusting the References Cited in the file of thispatent UNITED STATES PATENTS

1. A METHOD OF ELECTROSTATIC ELECTROPHOTOGRAPHIC REPRODUCTION OF IMAGESWHICH COMPRISES ELECTROSTATICALLYCHARGING THE OUTER SURFACE OF A SHEETOF INSULATING MATE RIAL WHILE ITS UNDER SURFACE IS IN CONTACT WITH APHOTOCONDUCTIVE LAYER, PRODUCING AN ELECTROSTATIC IMAGE ON SAID OUTERSURFACE OF SAID SHEET BY EXPOSURE TO A LIGHT IMAGE, APPLYINGELECTROSTATICALLY ATTACHABLE POWDER TO THE CHARGED OUTER SURFACE OF SAIDSHEET, REMOVING SAID SHEET FROM SAID PHOTOCONDUCTIVE LAYER, AND THENPERMANENTLY AFFIXING THE POWDER TO SAID SHEET OF INSULATING MATERIAL.